It’s so pretty! They’re caused by ice crystals in cirrus clouds bending sunlight toward you, and can only occur when the Sun is higher than 58° above the horizon. I was out right at noon, so the Sun was about 70° up—the Sun only gets that high at my latitude (almost exactly 40° north) in the summer around noon.

This one was patchy due to the bigger clouds blocking the cirrus, but I could see the arc extending well off to the left and right. It looked pretty much like you see it here; I enhanced the contrast a bit in the picture to bring out the colors, but it was nice and vivid by eye (which is why they’re sometimes called fire clouds).

A little bit closer in shows the colors more clearly.

Photo by Phil Plait

The Atmospheric Optics site says circumhorizonal arcs aren’t very rare, but I’ve only seen them a few times.* Maybe where you live they’re more common.

Phil Plait writes Slate’s Bad Astronomy blog and is an astronomer, public speaker, science evangelizer, and author of Death From the Skies!

But you won’t know unless you look up! I’m not saying don’t watch where you’re stepping (everyone I know like me who stargazes has a story about stumbling over something because they were looking up, not down). But once you have your footing, it pays to check out the sky. I’ve seen meteors, satellites, weird clouds, rockets re-entering (seriously), and just about every kind of bizarre rainbow-colored optical effect there is. And all it takes to broaden your horizon is to glance skyward. I recommend it.